Drill pipes are conventionally used in a string of such pipes connected together for purposes of drilling oil wells and the like. After such use, the string is removed and the pipes are successively disconnected from the string and laid aside for future reuse. During use a drill pipe may be subjected to excessive torque, with the result that it takes a permanent deformation that can lead to subsequent breaking of the pipe when it is again subjected to heavy strain. Since a pipe breaking while it is part of a string will cause a great deal of trouble and expense, it has long been desirable to find a practicable way of identifying those pipes which have been twisted excessively so that they will not be reused when new strings are assembled.